Sims 3 freezes after about 15-20 min.

Lately when I play Sims 3 the game freezes and doesn't respond. I just got another EP for the Sims 3 (Supernatural), halfway through making my Sim the game freezes, the mouse still moves but the music stops, no sound effects. Just frozen. I clicked ctrl shift c to see if I could do anything but nothing hgappened. So I pressed ctrl alt del where I appeared at this little menu where I could shut down the computer, log off etc., and then clicked cancel. The game was un-frozen and the bar from me clicking ctrl shift c was there and everything seemed normal. Then it froze again. So I'm guessing as it freezes the game itself keeps going, the image just doesn't respond? I just need help and don't know what to do.    

Also, it sounds like this just started after you got Supernatural installed? If so (and we find it's not caused by something else) you may want to start thinking about which expansions you have that you really enjoy using, and uninstalling one or two of them.